During a 24-hour period, the electrical grid experiences a large variation in demand. Electrical power consumption peaks during the day and dips significantly at night. Energy storage is often employed to mitigate and smooth out these large fluctuations in power demand. There are many ways of storing energy including electrical, chemical, thermal and mechanical means. Of these, thermal energy is an effective method to store energy in the form of heat or cold for use in heating, refrigeration and air conditioning applications. Considering the fact that a majority of the peak demand is generated by power-hungry heating and refrigeration appliances, thermal energy storage stands to become a leading contender in grid storage applications.
In countries such as India, thermal energy storage can also be used to mitigate the unreliable grid. By way of example, in many rural areas of India grid electricity is only available for a limited time during the day or night. In these situations, a thermal energy storage system can be charged when the grid is on and discharged when the grid is off to provide constant power for critical applications.
One such application is a village-based milk chiller incorporating a thermal energy storage system. The chiller can be operated in remote villages, requires only 5-6 hours of grid electricity to charge and, most significantly, does not require a regular backup diesel (or other) generator. Once charged, the system can quickly cool large amounts of raw milk to preserve its freshness and eliminate spoilage. In these situations, thermal energy storage is not only used to increase energy efficiency, but is essential to mitigate the unreliable grid supply while avoiding the use of expensive and polluting fossil fuels.
Thermal energy storage systems are typically designed for specific applications and are exactly matched for those applications. An example of a cold thermal energy storage system is an ice-bank tank which is typically designed for and fully integrated into an end-user application such as milk chilling. The prior art includes many examples of ice-based milk cooling systems dating back as far as the 1950s, such as U.S. Pat. No. 2,713,251. In this disclosure, the ice-based energy storage system is built into and is an integral part of the milk cooling tank, therefore cannot be easily separated and used for other refrigeration applications. Another example of a monolithic thermal energy storage system for cooling applications is the ice-on-coil storage system used in commercial HVAC applications such as disclosed by Gilbertson et al. in U.S. Pat. No. 5,090,207. The storage system described by Gilbertson et al. is a large monolithic system that requires custom designs and significant civil engineering effort to adapt to other applications.
To increase adoption of thermal energy storage and facilitate ease-of-use, it is desirable that the thermal energy storage be designed as a modular and compact component that can be added to (or subtracted from) any cooling or heating application according to the expected load on the system. Furthermore, thermal energy storage systems can be provided with well-defined specifications such that designers or users can incorporate them easily into their cooling or heating applications. An analogy to this is the electrical battery storage system. Designers can easily connect one or more electrical batteries in series or parallel to build a battery bank for a wide range of applications, from simple off-grid lighting systems to complex electrical vehicle storage systems and large solar power storage systems. This wide variety of electrical storage applications is facilitated by the modularity, compactness and well-defined specifications of the common electrical battery, such as the car battery.
Likewise, it is desirable to provide thermal energy storage systems that are generally as flexible, and as easy to build, as electrical storage systems. To achieve this, it is desirable to provide a compact and modular thermal energy battery with appropriate features to store and release thermal energy at a constant temperature and at a constant rate of discharge. These two specifications (temperature and rate of discharge) can become part of a standard set of specifications of a thermal energy battery which can be adopted by any manufacturer of such batteries.
U.S. Pat. No. 7,225,860 discloses a compact heat battery comprising of a cylinder containing encapsulating tubes filled with a phase-changing material (PCM) that absorbs and releases thermal energy. This battery uses maximally-packed PCM tubes to provide sufficient surface area to achieve a desired discharge rate. Disadvantageously, no provision is made for maintaining a constant discharge rate, other than having sufficient surface area for heat transfer. This is a common and well known method described in prior art, but it makes the battery less compact than it can otherwise be. Furthermore, by relying only on surface area for heat transfer, the battery will not be able to maintain a constant output during discharge because the heat exchange surface area becomes smaller as the PCM begins to melt.
U.S. Pat. No. 4,403,645 describes a high performance thermal storage apparatus which stores and releases its energy more efficiently using one long spiral tube rather than a plurality of PCM-filled encapsulants. However, the system is not modular, is difficult to build and cannot be easily sized for other applications. In another attempt at increasing performance, U.S. Pat. No. 7,503,185 describes a method for enhancing heat exchanging capability using ice-based thermal storage system. However this is a very expensive method of forming ice on copper tube coils.
Various methods of making thermal energy systems more modular are found in prior art, such as U.S. Pat. Nos. 5,239,839 and 4,827,735 which describe methods of encapsulating the PCM into expandable plastic tubes or quilts that can be modularly arranged and therefore used to build compact batteries of any size. These devices suffer from the same limitation as they can not maintain a constant output and discharge rate for long periods of time.
U.S. Pat. No. 4,524,756 describes a thermal energy storage system using modular batteries. This system does not use phase-change materials and thus cannot be very compact. Furthermore, the system described in this disclosure is limited to heat storage and cannot be easily adapted to refrigeration and air conditioning applications. A modular approach suited to refrigeration applications is described in US Patent 2002/0007637, but this method is expensive and relies on fixed path-ways that can only be changed manually using expensive quick-disconnects.
It would be desirable to provide a system that combines the dual demands of compactness and modularity to build efficient thermal energy storage banks that can be easily adapted to a variety of heating and cooling applications.